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Old 08-15-2011, 07:38 AM   #1
Rymantas
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2011 WRX Sedan
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Lightbulb Weird Knock Event during driving 2011 WRX

I've read around and heard that our knock sensors pick up some strange events (ambient noise in engine bay). I have been getting a knock event that was very consistent on the logs. They were always spaced equally apart while driving at highway speed (70 mph to be exact) with cruise control on. The events would happen every 45 seconds or so. I thought maybe I had bad gas at first but as I looked at my logs i began to see a pattern. The knock events always occurred when the AC compressor would kick on. So I tested this by making sure recirc was on, AC was off, and blower was off. I drove 100 miles with no knock. So then I drove another 100 miles (distance to work and home) with the AC on and it knocked every time the AC compressor came on.

I then tried forcing it to happen by randomly turning the AC on and off. It would actually trigger a knock event while moving 2 out of 3 times i would do this.

I will be going through the engine bay and checking to make sure everything with the AC system is snugged up. Has anyone else had occurrences like this?
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Old 08-15-2011, 12:09 PM   #2
D0nets
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Yea check the engine bay and make sure nothing is loose. Are you running a custom tune or anything? What load and rpm are the knock events at? If they are at really low load, you could modify the Feedback Correction Min Load table.
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Old 08-15-2011, 05:54 PM   #3
Rymantas
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Running Cobb OTS Stage 1 93. Occurs at 2800 rpm Highway cruising with cruise control on flat surface.
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Old 05-06-2013, 01:01 AM   #4
kabz
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Ok, I worked out that my stock 2013 WRX is doing this. A good chunk of the time, the AC compressor coming on creates a FBKC event, which decays over about 4 seconds.

This was really driving me nuts.
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Old 05-06-2013, 10:00 AM   #5
Cobb Tuning
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Quote:
Originally Posted by Rymantas View Post
I've read around and heard that our knock sensors pick up some strange events (ambient noise in engine bay). I have been getting a knock event that was very consistent on the logs. They were always spaced equally apart while driving at highway speed (70 mph to be exact) with cruise control on. The events would happen every 45 seconds or so. I thought maybe I had bad gas at first but as I looked at my logs i began to see a pattern. The knock events always occurred when the AC compressor would kick on. So I tested this by making sure recirc was on, AC was off, and blower was off. I drove 100 miles with no knock. So then I drove another 100 miles (distance to work and home) with the AC on and it knocked every time the AC compressor came on.

I then tried forcing it to happen by randomly turning the AC on and off. It would actually trigger a knock event while moving 2 out of 3 times i would do this.

I will be going through the engine bay and checking to make sure everything with the AC system is snugged up. Has anyone else had occurrences like this?
If it is a single count correction (-1.4) or so, I would not worry about it too much. The engine control unit (ECU) actually has a delay in which it prevents a correction to feedback knock taking place shortly after the A/C compressor comes on (a tuneable table in our Accesstuner tuning software). It may be something loose as mentioned (which is rattling as compressor comes on) or it may be that that some false knock due to A/C compressor coming on is normal at times (I haven't specifically check for a connection between A/C and feedback knock before via logging so I can't say for sure).

Bill
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Old 05-06-2013, 12:07 PM   #6
endrswrd
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I have seen this on many cars, mostly 08+. I usually only see single to double counts registering in conjunction with the A/C compressor, and it always unlearns quickly.
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